Abstract

Early observations with the James Webb Space Telescope have highlighted the excess of UV-bright galaxies at z > 10, with a derived UV luminosity function (UVLF) that exhibits a softer evolution than expected. This unexpected trend may result from several proposed mechanisms, including a high star formation efficiency (SFE) or a bursty star formation history (SFH). In this work, we aim at characterizing the burstiness level of high-redshift galaxy SFHs and its evolution. We implement a stochastic SFH module in cigale using power spectrum densities, to estimate the burstiness level of star formation in galaxies at 6 < z < 12. We find that SFHs with a high level of stochasticity better reproduce the SEDs of z > 6 galaxies, while smoother assumptions introduce biases when applied to galaxies with bursty star-formation activity. The assumed stochasticity level of the SFH also affects the constraints on galaxies’ physical properties, producing a strong and tight relation between the SFR and stellar mass in the case of a smooth SFH, down to a weak relation at z ≥ 7 for an SFH with a high level of stochasticity. Successively assuming different levels of burstiness, we determined the best-suited SFH for each 6 < z < 12 galaxy in the JADES sample from a Bayes Factor analysis. Galaxies are classified according to their level of burstiness, and the corresponding physical properties are associated to them. For massive galaxies (log M⋆/M⊙ ≥ 8.8), the fraction of bursty galaxies increases from 0.42±0.08 to 0.76±0.20 at z ∼ 6 and z ∼ 12, respectively. At all redshifts, only < 20% of low-mass galaxies are classified as bursty, due to their faintness resulting in low S/N. For bursty galaxies, the log10(SFR10/SFR100) ratio, another indicator of bursty star formation, does not evolve with redshift, but the fraction of galaxies with high log10(SFR10/SFR100) slightly increases from 0.25±0.06 to 0.37±0.11 between z ∼ 6 and z ∼ 9. We include additional constraints from observations on σUV and SFE, finding a maximum of 0.72±0.02 mag and 0.06±0.01, for σUV and SFE, respectively. This confirms that neither alone is responsible for the weak evolution of the UVLF at z > 10. Our results add further evidence that a combination with other mechanisms is likely responsible for the high-z UVLF